In this post I will bash Thunderbird. However, it is my mail client of choice, so even though it is somewhat crappy it is better than everything else.
On OS X: sometimes attachments don’t work correctly.
My best An attempt at an explanation is that, when viewing multiple emails with attachments in succession, and trying to open each attachment (e.g., PDFs from a network scanner) by double-clicking on the attached file, sometimes the attachment of the previously viewed email opens instead.
In Linux and on OS X: Auto-checking for new mail in IMAP folders (e.g., I have sieve script that auto-sorts certain emails) sometimes doesn’t work. When it fails, I have to click on the folder before Thunderbird will realize that it contains new messages. Today it actually crashed, and then when I restarted Thunderbird the two folders that get the most mail were re-downloaded in their entirety. This is the first time Thunderbird has actually crashed in a way that I thought might be related to this issue (which is good -> crashes are easier to debug than subtle logic problems).
Anyways, just venting. It would be schweeeet if these bugs got tracked down and fixed. If I had a time machine, I would attempt to do it myself.